Keys To Intraoperative Positioning For Subtalar Joint Procedures
Subtalar joint arthrodesis and subtalar joint arthroereisis are widely used surgical procedures for eliminating motion or limiting excessive pronation at the subtalar joint.1-3
Although the indications for either procedure may be vastly different, the successful outcome of both relies on proper positioning of the subtalar joint. A poorly positioned fusion or arthroereisis may result in continued pain and disability, gait dysfunction, or excessive stress on adjacent joints.4,5 Careful preoperative and intraoperative planning are required in order to ensure the best possible outcome.
Podiatric physicians should perform a thorough preoperative biomechanical evaluation on every patient they are considering for a possible subtalar joint arthrodesis or arthroereisis.6-8 Paramount in this evaluation is to determine the optimal position of the heel relative to the ground. When the patient is lying on the operating table, it is often very difficult for the surgeon to determine the proper heel position if he or she did not first obtain a weightbearing, preoperative reference point. Without this point of reference, the surgeon may fail to take into account frontal plane deformities of the hip, knee and forefoot that may ultimately result in an undesirable position of the heel.
For subtalar joint fusion, one should ensure positioning of the heel in approximately 2 to 4 degrees of valgus relative to the ground in order to maximize the stability of the medial column and prevent rigidity and overload of the lateral column. When it comes to arthroereisis, surgeons should place the heel in a position that is approximately 4 to 6 degrees less everted than its maximally pronated position relative to the ground. This position allows some compensatory pronation for shock absorption while still therapeutically limiting the excesss pronatory motion of the subtalar joint.
Fusing the heel in a varus position or overcorrection of an arthroereisis implant will prevent normal plantar loading of the medial column, and lead to lateral column overload. Conversely, too much valgus or under-correction of arthroereisis can lead to calcaneofibular impingement or less effectiveness of the implant correction in providing the desired effect. Accordingly, success or failure of either procedure depends heavily on obtaining a correct position of the heel relative to the ground.9,10
A Guide To Pertinent Measurements In The Preoperative Biomechanical Exam
During the preoperative biomechanical exam, the patient should be weightbearing in order to measure the neutral calcaneal stance position (NCSP), relaxed calcaneal stance position (RCSP), and the ideal final position of the heel relative to the ground (i.e. 2 to 4 degrees of valgus for fusion and 4 to 6 degrees less eversion than in maximum pronation for arthroereisis).
With the patient standing and the posterior heel facing the examiner, one should draw a line on the distal, posterior leg, parallel to the longitudinal axis of the tibia. The examiner places the patient in a neutral calcaneal stance position and marks the posterior heel in line with the longitudinal axis of the tibia. Then shift the patient to a maximally pronated (everted) position and mark the posterior heel to reflect the eversion of the heel. This is usually the relaxed calcaneal stance position.
The final mark represents the ideal position of the heel relative to the ground. This almost always falls between the two previous marks of both the neutral position and maximally pronated position.